Zeolite LZ-202 is a synthetic aluminosilicate molecular sieve topologically related to the mineral zeolite mazzite and to other synthetic zeolites of the mazzite type including zeolite Omega, or ZSM-4, as it is sometimes referred to. The synthesis techniques and characterization of LZ-202 are reported in detail in U.S. Pat. No. 4,840,779, issued Jun. 20, 1989, to T. R. Cannan, the entire disclosure of which is incorporated by reference herein Zeolite LZ-202 and mazzite, unlike zeolite Omega or ZSM-4, are crystallized hydrothermally from a reaction mixture free of organic templating agents. In a preferred method for synthesis of LZ-202, the procedure comprises (a) combining, with sufficient agitation to maintain a slurry, (i) an aqueous solution of an aluminum salt containing the anion of a strong mineral acid such as SO.sub.4.sup.-2 and (ii) an aqueous solution of an alkali metal hydroxide in sufficient amount to neutralize the aluminum salt of solution (i); (b) blending with the slurry of step (a) an alkali metal silicate in an amount such that the silica is from about 5 to about 30 times the moles of alumina and the alkali metal is from about 1 to about 12 times the molar concentration of alumina; (c) adjusting the water content of the reaction mixture, if necessary, to contain from about 30 to 100 moles of water per mole of alkali metal (as the oxide), and (d) digesting and crystallizing the mixture at a temperature of about 90.degree. C. to 150.degree. C. until crystals of LZ-202 are obtained. In terms of mole ratios of oxides, the reaction mixture formed from Na.sub.2 O, Al.sub.2 O.sub.3, SiO.sub.2, H.sub.2 O and SO.sub.4.sup.-2 ions contains the following proportions:
______________________________________ C A B Particularly Composition Preferred Preferred Ratios Ratios Ratios ______________________________________ SiO.sub.2 /Al.sub.2 O.sub.3 5-30 6-10 8.0-8.2 Na.sub.2 O/SiO.sub.2 0.03-1 0.2-0.5 0.30-0.34 H.sub.2 O/Na.sub.2 O 3.0 .+-. 0.2 3.0 .+-. 0.2 3.0 .+-. 0.2 SO.sub.4 /Al.sub.2 O.sub.3 3.0 .+-. 0.2 3.0 .+-. 0.2 3.0 .+-. 0.2 ______________________________________
The as-synthesized LZ-202 typically has a chemical composition (anhydrous basis) in terms of molar oxide ratios of: EQU 1.0.+-.0.5 Na.sub.2 O : Al.sub.2 O.sub.3 : 5-20 SiO.sub.2
and has an x-ray powder diffraction pattern containing as the principal d-spacings the lines set forth in tabular form below. The relative intensities are designated as VS (very strong), S (strong), MS (medium strong) and M (medium).
______________________________________ Relative d (A) Intensity ______________________________________ 15.54 M 9.06 VS 7.83 M 6.81 MS 5.93 M 4.68 M 3.79 MS 3.70 M 3.61 M 3.51 S 3.15 S 3.08 M 3.02 M 2.91 S ______________________________________
Except for the mineral species mazzite, LZ-202 can be differentiated from other omega-type zeolites in two ways, namely, the capability of LZ-202 to be completely ion-exchanged in its as-synthesized form without a prior calcination, and the ability to retain at least 30%, and usually 75%, of its crystallinity after undergoing treatment with an aqueous ammonium fluorosilicate solution to dealuminate its crystal lattice and substitute extraneous silicon atoms for framework aluminum atoms in accordance with the procedure described in U.S. Pat. No. 4,503,023, incorporated herein by reference. As used herein and in the appended claims the term "mazzite-type zeolite" refers to zeolites, such as LZ-202, having the mazzite crystal structure and crystallized from reaction mixtures essentially free of amine or quaternary ammonium templating agents. Unless otherwise specified, the mazzite-type zeolite is in its as-found or as-synthesized form.
In our commonly assigned copending Application Ser. No. 628,830, filed Dec. 17, 1990, there is disclosed a process for modifying forms of zeolite Omega synthesized from reaction systems containing organic templating agents. The process involves (a) an initial calcination in air to destroy the organic moieties, (b) ammonium ion-exchange to lower the alkali metal content of the air-calcined product, (c) calcination in steam and (d) ion-exchange using an aqueous ammonium ion solution adjusted to a pH of less than 4.0. The modification procedure improves the usefulness of the zeolite Omega product as a catalyst or catalyst base. In particular, the process is directed toward improving the thermal stability of activated forms of zeolite Omega, the instability believed to be related to the initial presence and subsequent removal of the organic cations. For that reason it was expected that neither mazzite nor LZ-202 would benefit from the proposed treatment since neither contain organic cations in the as-synthesized form.
Since the mineral mazzite and the as-synthesized form of LZ-202 contain essentially 100 equivalent percent alkali metal cations in charge-balancing association with AlO.sub.2.sup.- tetrahedral units, it is necessary to very substantially reduce the alkali metal cation content in order to use the zeolite as catalyst constituents in most of the common hydrocarbon conversion reactions. It has been surprisingly found, however, that ion-exchange of the as-synthesized LZ-202, while effectively removing the undesirable alkali metal cations, can have a marked effect upon the adsorptive properties of the zeolite which appear to be somewhat dependent upon the pH of the ion-exchange medium, but is in general unpredictable Regardless of the pH of the ammonium ion-exchange medium employed, the nitrogen surface area of the exchanged product is not more than about 100 m.sup.2 /g. indicating an unacceptably low degree of access to the internally-contained catalytically active sites of the zeolite structure for molecular species of even very small kinetic diameters. More significant is the adsorptive capacity of the ammonium-exchanged product for SF.sub.6 which has a kinetic diameter of 5.5.times.10.sup.-4 micrometers. The higher the SF.sub.6 adsorption capacity the larger the percentage of the crystal internal surface available to hydrocarbon or other organic species involved in commercial hydrocarbon conversion reactions. As illustrated by the experimental data set forth hereinafter, the initial ammonium exchange procedure can cause a modest increase in the SF.sub.6 capacity of the product, leave it substantially unchanged or reduce it, even to a value of essentially zero.